The present invention generally relates to packages adapted to house a microelectronic circuit device such as a silicon based semiconductor die. More particularly, the invention relates to a metal package having components with improved thermal and electrical performance.
Hermetically sealed packages for integrated circuit chips are typically comprised of a base component and a lid component which when bonded together define a cavity therebetween. The microelectronic circuit device is housed within the cavity. A leadframe is typically disposed between the base and lid and electrically interconnects the microelectronic device to external circuitry such as a printed circuit board. The integrated circuit is electrically connected to the leadframe by relatively small diameter gold or aluminum wires. Alternatively, connection between the chip and leadframe may be effected by tape automated bonding (TAB) techniques. In TAB, thin strips of copper or a copper alloy foil provide the necessary electrical interconnection. The leadframe further has a plurality of legs protruding outwardly from its central portion. The legs extend beyond the perimeter of the base and lid to provide terminals leading from the package.
Normal operation of the integrated circuit in the chip generates heat within the package. As the temperature of the typically silicon based microelectronic device increases, the internal resistance of the device also increases leading to degradation of the electrical performance of the device. Diffusion of metallic species such as from the circuitry metallization is increased further shortening the usable life of the device. As a general rule, every 10.degree. C. increase in device temperature shortens the usable life of the device by 50%.
In addition to shortening the usable life of the microelectronic device, as the temperature of the device and package components increases, so do thermally induced mechanical stresses. If the package base has a relatively high coefficient of thermal expansion, for example when copper or copper based package components are employed, the stresses on the device are severe enough to lead to fracture of the silicon based die. Thus, dual objectives of integrated circuit package design are to incorporate features in the package which minimize thermal expansion and remove heat.
To minimize thermally induced stresses, the components should be fabricated from materials having thermal expansion coefficients approximately matching that of the microelectronic device. To reduce the accumulation of heat within the package, those package components which dissipate heat, such as the base in a hermetically sealed package and the leadframe and optionally a heat sink or heat spreader in a non-hermetic plastic package, should be made from materials having high thermal conductivity. Further, some of these same components, such as the leadframe form a part of the conductive path of the package and must have suitable electrical conductivity.
One prior art integrated circuit chip package, commonly known as the CERDIP for ceramic dual-in-line package, is basically a ceramic package. The lid and base components are both alumina. The coefficient of thermal expansion of Al.sub.2 O.sub.3 is quite low and stresses to the electronic device are minimized. However, the ceramic base has high thermal resistance. The CERDIP is inadequate as a medium for carrying away and dissipating the heat generated by large scale integrated circuits or power devices.
In another prior art integrated circuit chip package, the base is constructed of metal, preferably copper or a copper based alloy. Metals are highly effective for thermal dissipation but have relatively high coefficients of thermal expansion. A severe mismatch with the silicon chip is often experienced. An additional insert material such as a buffer must be interposed between the chip and the base component. The buffer adds cost to the package and reduces thermal conduction from the chip to the package base.
Yet another package known in the art is the molded plastic package. A microelectronic device is attached to a metal bond pad and electrically interconnected to a leadframe. This assembly is then encapsulated within a molded plastic resin such as an epoxy. The thermal dissipation capability of the plastic resin is quite poor so thermal dissipation occurs by way of the leadframe or a heat sink or heat spreader molded into the package. If the leadframe and heat sink are fashioned from a low expansion metal such as alloy 42, inadequate heat dissipation is achieved. The use of a high expansion metal such as copper results in suitable heat dissipation, but stresses resulting from the high coefficient of thermal expansion of the copper often lead to fracture of the package or the device.
From the drawbacks of the constructions of the above packages, one can readily appreciate that the dual objectives of minimizing or matching thermal expansion and removing heat have been difficult to attain in practice. Consequently, a need still remains for a way to achieve these objectives.